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Common Stock (Common Stock Rollfoward) (Details) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Increase (Decrease) In Common Stock Shares [Roll Forward] |  |
Balance, January 1 | 228,770,000 | 210,020,000
Conversion of Convertible Debentures | 10,000 | 150,000
Issuance of common stock | 450,000 | 14,610,000
Issued for cash under Purchase Plans at market rate | 4,870,000 | 3,890,000
Discount on shares purchased under Dividend Reinvestment Plan | 0 | 0
Exercised | (20,000) | (100,000)
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period | 0 | 0
Balance, December 31 | 234,120,000 | 228,770,000
Increase (Decrease) In Common Stock Value [Roll Forward] |  |
Common Stock, Value, Issued, Beginning Balance | $ 5,601 | $ 4,738
Conversion of Convertible Debentures | 0 | 6
Issuance of common shares | 22 | 680
Issued for cash under Purchase Plans at market rate | 200 | 182
Discount on shares purchased under Dividend Reinvestment Plan Value | (9) | (9)
Options exercised under senior management share option plan | 1 | 3
Stock-based compensation | 1 | 1
Common Stock, Value, Issued, Ending Balance | $ 5,816 | $ 5,601
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum | 10.00% |
Employee Stock Option [Member] |  |
Increase (Decrease) In Common Stock Shares [Roll Forward] |  |
Exercised | (23,800) |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Common Stock, Capital Shares Reserved for Future Issuance | 1,000,000 | 1,300,000
Stock Compensation Plan [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Common Stock, Capital Shares Reserved for Future Issuance | 6,500,000 | 6,500,000
Dividend Reinvestment [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Common Stock, Capital Shares Reserved for Future Issuance | 12,600,000 | 4,200,000

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